The market for solid-state imaging apparatuses has expanded remarkably following the wide use of the digital cameras and mobile phones with cameras. In recent years, the demand for thinner digital cameras is getting stronger. Stated differently, this means that the focal length of a lens used for the camera module becomes shorter, and that light enters a solid-state imaging apparatus at a wide angle (that is, an angle obtained when measuring from a vertical axis of an incident surface of the solid-state imaging apparatus is wider).
In solid-state imaging apparatuses such as charge coupled device (CCD) image sensors or complementary metal oxide semiconductor (CMOS) image sensors, semiconductor integrated circuits having light-receiving parts are arranged in a two-dimensional array to convert a light signal from a subject into an electric signal. The sensitivity of the solid-state imaging apparatus is defined based on an amount of output current of a light-receiving element to an amount of incident light. Therefore, guiding the incident light surely into the light-receiving element is an important factor for improving the sensitivity.
In order to achieve this, it is required to improve light-collection efficiency of a microlens formed on the top portion of the CCD image sensor and the CMOS image sensor. The existing microlens is a resin spherical lens, and is used for almost all the solid-state imaging apparatuses including the CCD image sensors and the CMOS image sensors.
FIG. 10 shows an example of a basic structure of a pixel in a conventional solid-state imaging apparatus. A solid-state imaging apparatus 500 includes a microlens 501, a color filter 502, wire layers 503, a light-receiving element 506, and a Si substrate 507. As shown in FIG. 10, incident light 502A (dashed lines) which enters vertically into the microlens 501 is separated into colors using any one of the red (R), green (G), and blue (B) color filters 502, and then enters into the light-receiving element 506 without being affected by the light-blocking effect by the wire layers 503, and is converted into an electric signal at the light-receiving element 506.
FIG. 11 shows an example of a structure of a peripheral pixel in the conventional solid-state imaging apparatus. In peripheral pixels of the solid-state imaging apparatus 510, the incident angle of the incident light 502B (solid line) is large, and the wire layers 503 is shifted (shrunk) inwardly in an attempt to improve the light-collection efficiency.
However, in a fine pixel or a short-focal-length optical system such as miniature cameras where the incident angle of the incident light in the peripheral pixels is increased significantly, there is a problem that the circuit cannot be shrunk any more.
In order to address the above-described problem in wide-angle incidence to the peripheral pixels, Patent Literature (PTL) 1 proposes a solid-state imaging apparatus in which a refractive index distribution light-collecting element having an effective refractive index distribution is formed by its structure which is substantially the same as or finer than the wavelength of the incident light. With the solid-state imaging apparatus disclosed in PTL 1 in which the refractive index distribution type light-collecting elements having different refractive indices in the center portion, intermediate portion, and the peripheral portion of the imaging region is provided, even when the light enters into the peripheral pixels obliquely at a great angle relative to a vertical axis of the incident surface, the incident light can be collected at the light-receiving element and the sensitivity equivalent to that obtained at the center portion of the solid-state imaging apparatus can be obtained.